Abstract
In the mid-2000s, the Gulf of Lion was faced with a real crisis in pelagic fisheries, mainly for anchovy (Engraulis encrasicolus) and sardine (Sardina pilchardus). The drop in landings of these two species was not caused by a drop in abundance but by a decrease in their size and body condition, as fish that are too small are less economically interesting for fishermen. Various hypotheses have been put forward to explain these changes in fish populations (epizootic disease, natural predation, overfishing), but only the hypothesis of bottom-up control seems likely. The fish seem to have changed their diet, now consuming smaller prey, supposedly with less energy. The aim of this thesis was therefore to determine whether changes may have taken place in the planktonic community over the last two decades, in terms of density, size or taxonomic composition, which could explain the changes in the diet of sardines and anchovies and their declining condition. As plankton data are limited in the Gulf of Lion, we first focused on the variability of different environmental key factors for the planktonic community dynamics. Our results indicate that environmental conditions have broadly changed in the Gulf of Lion, with a major change in the mid-2000s, affecting Chlorophyll concentration (with a regime shift in 2007), but also SST, upwelling, thermal fronts, Rhone flow (and in particular N and P nutrient inputs) and winter deep convection. These environmental changes may have affected plankton production and, consequently, the small pelagic fish community, which showed similar patterns of variation. Based on data from PELMED's summer oceanographic campaigns (in 2007-2008 and since 2014), several types of phyto-plankton community have been identified, some dominated by dinoflagellates, notably Gymnodinium sp and others by diatoms, mainly Chaetoceros sp, Leptocylindrus sp and Proboscia sp. Concerning zooplankton, the main communities identified were dominated by cladocerans such as Penilia sp or Evadne sp and others by copepods such as Claucocalanus sp, Oithona sp or Centropages sp. However, no strong links were observed either with the environment or with the condition of the fish and these different summer communities. Finally, the long-term variability of zooplankton was studied in the Ligurian Sea, revealing a high stability of the community in terms of density, size and taxonomic composition. Marked changes were observed in 2015 with a decrease in density of the smallest crustaceans as well as a decrease in density of gelatinous filter feeders or carnivorous, possibly related to the increase in temperature and changes in winter conditions. Thus, although no causal relationship could clearly be established between planktonic dynamics and small pelagics, environmental changes have indeed occurred in the Gulf of Lion, and the planktonic production of the shelf seems to have been impacted. Strong and punctual processes, as in 2015, seem to have a strong and rather durable impact on the planktonic communities. Although no clear conclusion can be drawn as to a possible bottom-up control of small pelagic populations, this hypothesis cannot be discarded. The main obstacle appears to be the lack of monitoring of the planktonic community in the Gulf of Lion, which it seems necessary to put in place in order to understand its dynamics in the face of climate change and the possible impact on small pelagics.